Multilevel converters can be used for rectifying AC to produce DC, and may also be employed to generate AC output voltages for use in motor drives or other power conversion systems. This modular form of converter finds particular use in situations where relatively large output voltages are required. Multilevel voltage source converter architectures include flying or switched capacitor designs (FC), neutral point clamped (NPC) designs, modular multilevel converter (MMC), as well as cascaded and hybrid typologies such as and the cascaded H-bridge (CHB) designs. NPC designs include a pair of capacitors connected across a DC input providing a neutral node, with each capacitor being charged to half the DC input value, and a series of switches are connected across the DC bus, with a pair of diodes connecting intermediate switch nodes to the neutral point. Multilevel converters offer certain advantages for medium-voltage high-power conversion applications; such as motor drives, micro-grids and distributed generation systems. The main features of these topologies, as compared with the two-level voltage source converters (VSC), are the capability to reduce harmonic distortion of the AC-side waveforms, to reduce dv/dt switching stresses, to reduce switching losses, and to minimize or even eliminate the need for an interface transformer. Certain variant and hybrid configurations have been proposed, including five-level H-bridge NPC (5L-HNPC), three-level active NPC (3L-ANPC), and five-level active NPC (5L-ANPC). Although these hybrid topologies mitigate some drawbacks of the classical multilevel topologies, certain shortcomings remain. For example, a 5L-ANPC is a combination of a 3L-ANPC and 3L-FC, which increases the number of levels to reach higher output levels. However, in addition to complexity of the system due to the need to control flying capacitor voltages to facilitate use of the same rating switch for all the switches, two devices are connected in series for the top and bottom switches since the voltage stresses of the switches for a 5L-ANPC are different, with the outer switch ratings being half of the dc-link voltage while the inner devices see only one third of the dc-link voltage.
Nested neutral point clamped (NNPC) multilevel designs address these shortcomings, such as a four-level NNPC converter, which can operate for a wide voltage range (e.g., 2.4-7.2 Kv) with all the switches experiencing the same or similar voltage stress levels without requiring series-connection of multiple switches. Moreover, NNPC architectures generally have fewer components than conventional multilevel converters and may mitigate the need for complex transformers. Certain of these advantages, moreover, are facilitated by controlling the flying capacitor voltages, thereby controlling the switching device voltages levels. However, implementing switched capacitor voltage control or other control objectives using space vector modulation (SVM) requires many calculations in each switching period, and thus increases the control system complexity and computational delays can deteriorate the performance of the control system. Accordingly, a need remains for simple, yet robust, control techniques and apparatus for operating NNPC and other multilevel converter architectures without the computational complexities of space vector modulation techniques.